Crabgrass is a common warm-season annual grass that invades lawns across the country, often signaling the start of the summer weed season. Its growth is governed by environmental conditions, particularly the warming of the soil, rather than a specific calendar date. Understanding the precise timing of its life cycle is the most effective tool for managing this persistent lawn pest. The entire existence of a crabgrass plant, from germination to death, occurs within a single growing season.
The Germination Trigger
The moment crabgrass begins to grow is entirely dependent on the soil temperature, not the air temperature or the calendar date. Germination starts once the soil reaches a consistent temperature range, typically between 55 and 60 degrees Fahrenheit. This temperature must be sustained for several consecutive days, usually measured at a shallow depth of two to four inches below the surface.
This temperature requirement is the biological trigger for seeds that have lain dormant over the winter. Relying on a fixed date for the start of spring is unreliable because local weather patterns vary significantly. A warm spell can initiate germination earlier than expected. While initial germination begins near 55 degrees, the majority of seeds sprout once the soil warms further, often reaching 60 to 70 degrees Fahrenheit.
The Annual Life Cycle
Once the seeds germinate in the spring, the crabgrass plant immediately enters its vegetative growth phase. As a warm-season annual, it is adapted to thrive during the hottest months of the year, typically from late spring through August. The plant exhibits a tillering growth pattern, producing multiple shoots or stems from the base, which gives it a dense, clumpy, crab-like appearance.
This aggressive, spreading growth allows it to outcompete cool-season turfgrasses during periods of heat and drought stress. A single crabgrass plant can produce hundreds of tillers, expanding outward and choking out the surrounding lawn. Its efficiency in hot, dry conditions is due to its C4 photosynthetic pathway, which allows it to utilize intense sunlight and high temperatures more effectively than many common lawn grasses.
Seasonal Decline and Seed Setting
As summer progresses, the plant shifts its focus from vegetative growth to reproduction, preparing for the end of its life cycle. This transition occurs when the days begin to shorten in late summer and early fall. The plant develops purplish or green seed heads that are often difficult to see below typical mowing height.
A mature crabgrass plant can produce a large number of seeds, with estimates reaching up to 150,000 viable seeds from a single plant. These seeds drop onto the soil surface, replenishing the seed bank for the following spring. Since the plant is an annual, it does not survive the winter; it is killed by the first hard frost of the fall, typically when overnight temperatures reach 32 degrees Fahrenheit.
Timing Control Measures
Effective control of crabgrass depends on intervening at the correct point in its life cycle. The most common preventative method involves using a pre-emergent herbicide, which must be applied before the germination trigger is met. This application window opens in the early spring, when soil temperatures are still below the 55-degree Fahrenheit threshold.
Applying the pre-emergent too early allows the product to break down before the seeds sprout, while applying it too late means the crabgrass has already begun to emerge. If plants have emerged, a post-emergent herbicide is necessary. This must be applied while the plants are still young and actively growing, as control becomes more difficult once the plants mature and begin to produce seeds in late summer.